The present invention relates to nano-manufacturing technology solutions involving equipment, processes, and materials used in the deposition, patterning, and treatment of thin-films and coatings, with representative examples including (but not limited to) applications involving: semiconductor and dielectric materials and devices, silicon-based wafers, flat panel displays (such as TFTs), masks and filters, energy conversion and storage (such as photovoltaic cells, fuel cells, and batteries), solid state lighting (such as LEDs and OLEDs), magnetic and optical storage, micro-electro-mechanical systems (MEMS) and nano-electro-mechanical systems (NEMS), micro-optic and optoelectronic devices, architectural and automotive glasses, metallization systems for metal and polymer foils and packaging, and micro- and nano-molding. More particularly, the invention relates to the application of thin films onto a surface. Even more particularly, the invention relates a method of formation of low-stress and high-optical-index films by chemical vapor deposition (CVD).
Though the invention has applications beyond this representative example, a synopsis of imaging sensor technologies will be helpful. Charge-coupled devices (CCDs) have appeared in imaging devices for over thirty years. The primary advantage being that they process a high percentage of the incoming light which is also referred to as having a high fill factor. Manufacturers would like to switch to CMOS image sensors which would be faster, more flexible, less power intensive, and manufacturable without specialized fabrication facilities. Increasing the detection efficiency of the CMOS image sensor would allow CMOS sensors to further displace CCDs from image acquisition applications. Miniaturizing the electronics integrated alongside the optically active regions is one way to increase the optical efficiency but results in an increase in manufacturing cost. Guiding more light to the optically active surface with optical elements would increase efficiency without creating a reliance on more expensive finer linewidth tooling.
FIG. 1A shows a cross-sectional view of an existing method of increasing the detection efficiency of CMOS image sensors. The detectors 102 are generally manufactured in a two dimensional array located at the interface between two materials, in this case a substrate material and a layer of glass. A matching array of lenses 114 can be placed above the detectors to help guide more light to the detector. Not shown are electrical connections for reading out the signals generated by the detectors. A line incident from above is shown indicating a path of illumination. The path bends upon entering the plano-convex silicon dioxide lens and executes a path 126 distinct from the dashed line. Without the presence of a lens, the illumination executes the path 132 which leads to an adjacent detector. This is undesirable because it results in a detected image which has a less crisp image.
Refractive and reflective optical elements typically employ an interface between two regions of differing refractive indices. The interface is usually smooth down to and including length scales similar to the working optical wavelengths. Making the regions including the interface free of defects helps reduce optical scattering which, in the case of image acquisition, may result in loss of signal and cross-talk between cells. Deposition techniques must be developed which support these design criteria.
Conventional thermal CVD processes supply reactive gases to the substrate surface where the heat from the surface induces chemical reactions to produce a film. Improvements in deposition rate and film properties have been achieved through the use of plasma sources to assist the chemical reactions. Plasma enhanced CVD (PECVD) techniques promote excitation and dissociation of the reactant gases by the application of radio frequency (RF) energy to a reaction zone near the substrate surface, thereby creating a plasma. The high reactivity of the species in the plasma reduces the energy required to activate a chemical reaction. This effectively lowers the substrate temperature required for PECVD processes as compared to conventional thermal CVD processes. Reducing the substrate temperature is attractive because it lowers the chances of diffusion or other mass transport effects which may cause a reduction in the yield of the manufacturing process.
Further improvements have been enabled by another plasma technique known as high density plasma chemical vapor deposition process (HDP-CVD). HDP-CVD allows the use of lower partial pressures of reactant gases while maintaining a higher ionic concentration. HDP-CVD also allows the accelerating energy to be controlled independently of the ionization energy, and enables the reactant ions to become both the reactive and bombarding species. This has been shown to result in improved gap-fill, therefore the technique is particularly important when forming films on patterned surfaces.
A material commonly used in the fabrication of integrated devices is silicon nitride. When used to create optical elements, the higher index of refraction compared to silicon dioxide provides the ability to control electromagnetic radiation of many wavelengths including those visible to the human eye. However, the high stress of traditionally deposited silicon nitride on a silicon base substrate can result in imperfections including particulates which may result from delamination.
As this optical example indicates, there remains a general need in the art for methods of depositing high density silicon nitride and related materials onto substrates with reduced film stress.